In the imaging arts, elements that can be imagewise exposed by means of light radiation are well known. The availability of infrared laser diodes has provided a convenient means of generating images onto a variety of substrates using a laser scanner. In particular, laser thermal transfer systems have gained significant attention over the past decade. In a typical laser thermal transfer system, a donor sheet comprising a layer of an infrared absorbing transfer medium is placed in contact with a receptor, and the assembly is exposed to a pattern of infrared (IR) radiation. Absorption of the IR radiation causes a rapid build-up of heat in the exposed areas which in turn causes transfer of the medium from the donor to the receptor to form an image. This transfer can result, for example, from sublimation (or diffusion), ablative transfer, film transfer, or mass transfer.
Sublimation or diffusion transfer systems involve a mechanism wherein a colorant is sublimed (or difflused) to the receptor without co-transfer of the binder. This process enables the amount of colorant transferred to vary continuously with the input of radiation energy. Examples of this type of process are discussed in JP 51-088016; GB 2,083,726; as well as U.S. Pat. Nos. 5,126,760; 5,053,381; 5,017,547 and 4,541,830.
In an ablative thermal transfer system, the exposed transfer medium is propelled from the donor to a receptor by generation of a gas. Specific polymers are selected which decompose upon exposure to heat to rapidly generate a gas. The build-up of gas under or within the transfer media acts as a propellant to transfer the media to the receptor. Examples of various laser ablative systems may be found in U.S. Pat. Nos. 5,516,622; 5,518,861; 5,326,619; 5,308,737; 5,278,023; 5,256,506; 5,171,650; 5,156,938; 3,962,513; and WO 90/12342.
In a mass-transfer system, the colorant and associated binder materials transfer in a molten or semi-molten state (melt-stick transfer) to a receptor upon exposure to the radiation source. The thermal transfer media sticks to the receptor surface with greater strength than it adheres to the donor surface resulting in physical transfer of the media in the imaged areas. There is essentially 0% or 100% transfer of colorant depending on whether the applied energy exceeds a certain threshold. Examples of these types of systems may be found in JP 63-319192; JP 69-319192; WO 97/15173; EP 530018; EP 602893; EP 675003; EP 745489; U.S. Pat. Nos. 5,501,937; 5,401,606 and 5,019,549.
In laser-induced film transfer (LIFT), the donor sheets contain a crosslinking agent that reacts with a binder imaging to form a high molecular weight network. The net effect of this crosslinking is better control of melt flow phenomena, transfer of more cohesive material to the receptor, and higher quality dots. Examples of this type of system may be found in U.S. patent application Ser. No. 08/842,151, filed on Apr. 22, 1997.
Ideally, the transfer media absorbs at a wavelength different from the imaging radiation. However, black colorants typically absorb over a broad range of wavelengths making it difficult to formulate a black donor that does not interfere with the imaging radiation. Absorption of infrared radiation by black colorants is particularly troublesome since the absorption of the infrared radiation causes additional heat generation which leads to poor image quality or in some cases may destroy the imaging media. Therefore, there is a need for a black formulation that does not interfere significantly with infrared imaging sources.